1. Field of the Invention
This invention relates to an optical beam scanning apparatus in which a light beam from a light source is reflected by a rotary polygonal mirror and then radiated to the scanned plane through a condensing lens for scanning of the beam.
2. Description of the Prior Art
The optical beam scanning apparatus of this kind is, as schematically shown in FIG. 1, so basically arranged that a light beam emitted from a laser 2 or other source and impinged upon a rotary polygonal mirror 1 is reflected by the mirror surface of the rotary polygonal mirror 1 rotating in the direction of an arrow and then passed through a condensing lens (f..theta. lens) 3 to form a light spot which moves at a uniform speed in the scanning direction (i.e., x-direction) along a scanning line SP1 on a scanned plane SP. However, if the respective mirror surfaces are not in parallel to the rotary shaft of the rotary polygonal mirror 1 and there is a variation in angles of the mirror surfaces with respect to the rotary shaft (i.e., a tilting angle error), the light spot is shifted nonuniformly in the direction perpendicular to the scanning direction (i.e., y-direction). As shown in FIG. 2, for example, assuming that tilting angle error of the reflection surface 1a is .DELTA..theta., there will be caused a shift amount of .DELTA.d=2f..DELTA..theta. on the scanned plane SP (where f is a focal length of the condensing lens 3). Such tilting angle error and vibrations of the rotary shaft will bring about a variation in pitch of the scanning lines, so some measure must be taken to eliminate the adverse effect. As one of such measures, it is considered that the machining accuracy can be improved to minimize tilting angle error. But in practice the machining accuracy has already reached near its limit, and even if more accurate machining can be made, there will arise additional several problems that many processes are required for machining, it is difficult to perform mass production, and the manufacturing cost becomes very high.
As shown in FIGS. 3 and 4, there has been also known heretofore another scanning apparatus such that a light beam is impinged upon the rotary polygonal mirror 1 in the form of a line parallel to the scanning direction by using a cylindrical lens 4, etc., and a cylindrical lens (or toroidal lens) 5 and the condensing lens 3 are disposed between the rotary polygonal mirror 1 and the scanned plane SP thereby to make the line beam and the scanning position to have an optical conjugate relation with respect to the direction at a right angle with the scanning direction, thus effecting correction (refer to Japanese Patent Laid-Open No. 49315/73). However, in the case of using the cylindrical lens 5 of a shape as illustrated in the figures, it is impossible to obtain a uniform spot size all over the scanning width. When a toroidal lens having the given curvature also in the x-direction is employed in place of the cylindrical lens 5, a substantially uniform spot size can be obtained. But the high cost of the toroidal lens will cause a new problem that the optical beam scanning apparatus becomes very costly.
In addition, such a conventional scanning apparatus is further known that, as shown in FIGS. 5 and 6, cylindrical lenses 6 and 7 are used to form a flat beam substantially parallel with the direction of the optical axis (FIG. 7 shows a beam shape on one mirror surface 1a of the rotary polygonal mirror 1), the beam is impinged upon the rotary polygonal mirror 1, and another cylindrical lens 8 is disposed between the condensing lens 3 and the scanned plane SP. This apparatus is also accompanied with the problem that it is impossible to obtain a uniform spot size all over the scanning width.